Equine protozoal myeloencephalitis (EPM) is a common neurologic disease in horses in the USA. It is not generally seen in the UK, except in imported horses.
EPM is a disease caused by infection with the protozoa Sarcocystis neurona or Neospora hughesi, but it is more commonly caused by N. hughesi which is carried by opossums in North and South America. It is found in horse feed, hay, pasture or water that has been contaminated with opossum faeces, and horses can be affected at any age. After a horse ingests the contaminated feed or water, the protozoa travels through the digestive tract and enters the bloodstream.
Some horses are immune to the protozoa and are able to clear it from the blood before it crosses the blood brain barrier, and the horse may carry an antibody to the protozoa for life. Exposure to EPM means it doesn’t have an active infection, and will not cause neurological symptoms.
However, when infection does occur, the protozoa cross the blood-brain barrier, and infect the central nervous system (CNS) causing neurological symptoms.
There is a wide range of neurological signs involving multiple site CNS signs, including weakness (most commonly of the hindlimbs), ataxia (incoordination), muscle wastage of the rump or shoulders (usually asymmetric), and signs of brain disease.
Other signs include unusual sweating patterns, head tilt, facial paralysis, lack of tongue tone, drooping ear, visual problems, behavioural abnormalities, dragging a hoof, carrying tail to one side and seizures.
Your vet will want to perform a neurological examination and take a sample of blood or cerebrospinal fluid (CSF) for testing.
Early detection is essential for successful treatment. With medication, horses can recover from EPM, however neurologic damage can be permanent. No drug kills 100% of the protozoa, they only reduce the population; the horse’s immune system has to do the rest!
Treatment can take 28 day to 6 months, depending on the drug treatment used by your vet.
Without treatment, EPM is often fatal.
Prevention is relatively difficult, but you can take appropriate steps to reduce your horse’s exposure to the organism.
The likeliest source of infection is opossum faeces, so horse owners should try to keep opossums away from their horses, especially from feeding and watering areas. Horse and pet feed should be stored in a suitable container away from opossums. Rubbish and open feed bags should be kept in closed galvanised metal containers. Avoid attracting scavenging opossums by removing any dead animals, eg cats, rats etc, from around the property where the horses are kept.
Opossums can also be trapped and relocated humanely from any affected areas of pastures or woodland.
A vaccine is available, but its effectiveness is still unknown.